


once, you were my hero.

by hovercraft



Category: Fate/Grand Order, Fate/stay night & Related Fandoms
Genre: Beauty and the Beast Elements, M/M, Sacrificial Bride AU
Language: English
Status: Completed
Published: 2020-02-14
Updated: 2020-02-14
Packaged: 2021-02-28 03:00:52
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,807
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22706605
Author URL: https://archiveofourown.org/users/hovercraft/pseuds/hovercraft
Summary: A monster in the wood demands a bride.Many have left, none have returned.It's up to a knight to disguise himself and slay the beast.
Relationships: Cú Chulainn Alter | Berserker/Diarmuid Ua Duibhne | Lancer
Comments: 7
Kudos: 93





	once, you were my hero.

**Author's Note:**

> This is part of a gift exchange for Valentine's Day with my friend! HI I HOPE YOU HAVE A GREAT ONE ;u;;;;

There was nothing, no creature so dangerous as the once Child of Light.

Rumors flew about him like vultures to a kill, that he’d been cursed, died young and reborn as a beast, or merely, a spirit inhabited his corpse that was no longer human. No one could imagine Cu Chulainn could be left behind in the husk of the monster that dwelled within a cavern covered in moss and the tangled roots of dying trees. He was a beast with jowls wet with blood, who didn’t care who he hunted. Humans, wolves, other animals… the rumors exaggerated, of course, but one night, a man came back claiming he’d spoken to the monster. It let him live in exchange for delivering a message.

He wanted a bride.

No one knew the true nature of Cu Chulainn now, but according to his account, he’d grown scales, claws, and a tail wrapped around his form, swaying with predatory intent. Everyone knew this once person had killed and eaten any kind of meat it could get his hands on—therefore, this was nothing more than a sacrifice. Any woman sent to him would surely be consumed, but so came the threat of the monster stealing away someone in the night. The local villagers sent one girl off into the woods, then another, then another.

None returned. Still, the monster repeated his request.

A bride.

Desperate not to let anyone else get killed, the village held a meeting.

One of them would contact the knights. Surely, they could kill the beast and bring peace back to their village.

Of all the knights, Diarmuid Ua Duibhne would answer their call and promise to slay the monster.

The plan was simple; disguise himself as a bride for the monster, then kill it either while it slept or before it attacked, whichever came first. There was a ceremony to dressing him, a white gown, a crown of queen’s lace, sleeves that were sure to cover his muscular form. He was to be lead to the edge of the forest by a procession that would thin out as time went on, for their own safety. As leaves crunched beneath his elegant shoes and as lace itched at his skin, he couldn’t help but wonder what had happened to the poor, young women who had been sent Cu’s way. There was always a chance they had escaped rather than be eaten, or maybe when their procession left them, they ran into the forest, never to be seen or heard from again. Surely Cu wasn’t… consuming them.

“Is it true that he eats… humans?”

“Every person that’s seen him and lived said he smelled of blood,” Said the last man left in the procession, carrying Diarmuid’s lances to hide them somewhere out of sight so that Diarmuid could retrieve them to slay the beast. “And people have definitely disappeared in search of him. Why he wants a sacrificial bride… no one can say. You look the part, well enough.”

Diarmuid nodded quietly, knowing there to be no ceremony when it came to this. He’d either be mauled, sent away, or claimed in the most vicious way possible. No—there was no thinking like that. He’d kill Cu Chulainn, and end his reign of terror. He saw the eyes of frightened women in the village—knowing any one of them could be next should he fail was inspiration enough.

The plan was as follows: depending on what Cu did, he would stay outside the mouth of the cave, try to lure him to where his lances lay in wait, and then fight it to the death. This was, of course, if Cu was without reason, deigned to eat his flesh or anything like that. People had faith in him, and he wouldn’t squander it. He knew the legends of old about the man, and while he couldn’t condone what he was doing now, a part of him ached for the fact that a hero like him had been reduced to this.

The mouth of the cave was enormous, and the last of Diarmuid’s entourage had left him behind. At first, there was no sight of anyone or anything. This would be the perfect moment for a bride left here to run, Diarmuid thought. Perhaps they all had?

Then came the sounds of movement within the cave, brushing up against its walls, footsteps that practically shook the ground beneath him. Diarmuid wouldn’t let himself be intimidated—he was here to kill the thing, not submit to it.

Cu was tall—impossibly tall as he reached the mouth of the cave, already big enough and yet he was hunched inside of it. When his walk turned into a stride, standing at his full height, Diarmuid wondered if he even _could_ be killed. He was fated to die to a boar, not a fallen hero, so he wasn’t too concerned—scratch that, he was incredibly concerned. As Cu glowered down at him, he sniffed the air once. Diarmuid was too caught up in looking at the sharp spines wrapped around his body, his legs, his feet and the long tail stretching out of his back to notice it.

“You’re no woman.”

“…” If he could just tell off of scent, then… there was no point in lying. “How many of those village girls have you killed?”

“None.”

“Then why haven’t they returned home?!”

“Have you noticed?” The deep, throaty voice continued. “These woods are impossible to navigate alone. Full of wolves. I suspect they became someone else’s meal.”

So they ran… Diarmuid glowered at him. “And you wouldn’t save one? Someone meant to be your bride?”

“If they’d rather die than become that, it’s not on me.” Cu said apathetically. “I’m not dragging them back kicking and screaming.”

Diarmuid was infuriated, but it became clear that the crime Cu was accused of wasn’t true. He had no reason to kill him. Just before he began to speak, Cu picked him up and threw him over his shoulder. Diarmuid thrashed, even at the threat of becoming impaled by one of the thorns wrapping around his shoulder.

“What are you doing--?!”

“A bride is a bride.”

“Are you insane? I’m a ma—”

But into the cave they disappeared.

\--

It was more comfortable inside than Diarmuid would have guessed.

A fire roared in the middle of it, with a small crevice in the roof of the cave allowing smoke to pour out. Furs of all kinds lined the interior, and of course, skeletal remains were littered in the corner, picked clean. Diarmuid tried to get a good look, but none of them seemed… human. Nonetheless, he was worried about where this would go. As Cu dumped him unceremoniously onto a pile of soft furs, Diarmuid scooted back. He wouldn’t be able to get his weapons in a place like this. More curious, though, was how Cu knelt and stared him in the eyes.

Diarmuid didn’t dare look away, but Cu’s stare was as threatening as it was intimidating. He wouldn’t back down, even if this man were to gore him, he refused to give him the pleasure of frightening him. It seemed Cu wasn’t interested in that, however.

He grabbed him, roughly, looking over his wrists before feeling up his arm to the elbow, like he was examining a piece of meat for tenderness. Diarmuid let him, if only not to pull the trigger too quickly on his anger. Cu’s large hands traveled up and down his waist, feeling him up for size before, in a swift motion, tearing the front of the dress apart as if it were wrapping paper. _That_ was when Diarmuid began to protest. He reeled back against the wall and planted his foot on Cu’s chest, not that it moved him any.

“I am not a gift from the village!”

“Oh? Then what are you.”

“…” It would be safer to let Cu continue, but Diarmuid had his pride, his honor. “I was sent here to kill you.”

“No weapons…” Cu drawled, feeling up his thighs for a hidden blade. “No strength. You came here on a suicide mission.”

“… I hadn’t anticipated you would actually want a bride. Everyone thought you were eating them.”

That actually got Cu to laugh, bearing his sharp teeth for but a moment. “Did they? I’ve been stealing livestock, but women? That’s hilarious. No meat on their bones.”

Diarmuid couldn’t tell if Cu was joking, but didn’t underestimate him. “What happened to you, Child of Light?”

Cu pondered the question, his smile leaving his lips. “Cursed.”

“By who?”

“Doesn’t matter.” Cu grabbed the leg that was driving into his chest and pushed it aside, unwrapping Diarmuid fully. He’d worn his green suit underneath it, making the monster sigh at the idea there were even _more_ clothes to take off of him. Diarmuid stared up at him, challenging him.

“Then why a bride?”

“… you’re noisy. You ask too many questions.”

“I think I have the right to know.”

“You have nothing.”

Still, Cu gave up on stripping him then and there, shifting away from him to prod at the fire. Diarmuid wondered if this was his chance to escape, but… no, he needed to know. Know why Cu had changed, if he was still a threat to the land.

“What do you want of me?” He asked, finally.

“… to be a bride. That’s it.”

“… Will you stop asking the village to send women here if I stay in their place?”

“Yes.”

Then that settled it. Until he found out why Cu needed him here so badly, Diarmuid would stay.

\--

Cu wasn’t so different from the legends. More sullen, definitely, but the wild air he was rumored to possess had never gone away.

To stop him from culling livestock from the locals, Diarmuid gathered up his lances and devoted himself to hunting. As apathetic as he was, to Cu, this was one of the few joys he had remaining, so he accompanied Diarmuid. Cu really was more beast than man, now—when Diarmuid scored the flesh and prepared it for cooking, Cu had instead stripped raw meat off the bones of the deer they caught and ate it like that—plain and simple, still warm with the residue of life. Diarmuid watched in fascination as he did it. To think someone who used to be so human… or at least, so much _more_ human, anyway, had become this.

Diarmuid would cook his meat with herbs he gathered from the woods and grimace slightly at the gamey taste. If there was ever a better time to try and get to know Cu, now seemed to be it.

“So, is there a way to break this curse of yours…?”

“…”

“I’m assuming that’s what you want a bride for, isn’t it?”

“…”

“A one-sided conversation helps no-one. Besides, someone like you wouldn’t pick up someone they didn’t have a use for—”

“Yes.”

“To the first or second question?”

“Both,” Cu tore off another piece of meat, this time going for what was roasting over the fire. Diarmuid finally had his answer—he wanted a bride in order to break his curse, which was sensible if those were the terms under which he’d been hexed. “A woman made it so I couldn’t change back unless I fell in love. She meant it to be with her, so I left.”

“That seems like a way of strong-arming it…”

“That’s one way to put it.”

So… Cu wanted a bride to fall in love with. The only problem was, the man was so densely apathetic and cruelly uncaring that such a thing felt impossible, especially if he was asking that of Diarmuid. Still, a sense of duty spurred him onward, thinking maybe if he wasn’t the right one, he could find someone for Cu.

Yeah, that sounded like it could work.

\--

As time moved forward and the mild spring turned into warm summer, Diarmuid had discreetly told the village of his mission and to leave them alone for now. He hunted with Cu, he ate with him, he slept in the same piles of furs as him. Little shimmers of the man of legend peeked out every now and then. This really was Cu Chulainn, albeit more sullen, both more and less feral as the original. Diarmuid couldn’t help but feel warm around him, to be in the presence of such a legend was awe-inspiring, even if he wasn’t much for words and seemed to wish for Diarmuid to solve his problem all on his own.

Diarmuid dismissed himself from his side to find somewhere to bathe. Rumor had it that there was a warm spring nearby, and sure enough, Diarmuid had found it. It would be the perfect opportunity to wash his clothes and the rest of his body with the soaps he’d borrowed from the village. Living in a cave full of pelts wasn’t the most… elegantly scented place in the world.

Diarmuid was almost halfway through a bar of soap, scrubbing his outfit clean of blood when he noticed a second presence. Cu was stripping off his clothing and wading out into the water beside him. Diarmuid tried not to stare, but he was entirely hard to ignore.

“If you want me to wash your clothes, pass them over here—”

“You don’t need to do that.”

“What are you here for, then?”

He was surprised to feel Cu’s hands slip around his waist, and by surprised, one might mean totally shocked.

“Wh--?!”

“Calm down. You’re my bride, aren’t you?”

“I thought I was—I was _temporary_ until we found someone suitable—”

“I don’t ever recall saying that.”

“… then it’s my fault for assuming,” Diarmuid said, finally, refusing to let go of the wet clothes he held in his hands, eventually relenting when Cu pulled him into his lap. Everything he did was ruthless manhandling, but this time, he was surprisingly gentle in comparison, even with Diarmuid’s back pressed against his chest. That still didn’t mean he was totally gentle, but still.

Diarmuid admired Cu, the legend of who he was, and his heart ached a million times over for the fact that he’d been cursed. But… there was nothing he could surely do about it, could he? Cu had tolerated his presence, called him a bride and didn’t seem keen on finding anyone else to do it. Curiosity piqued his interest. Cu wouldn’t dare say it, and Diarmuid couldn’t force such a thing, but maybe there was something he could do for him.

In a quick motion, Diarmuid turned to sit on Cu’s lap, straddling it. The other man looked at him with a quiet interest, as if wanting to see where he’d go from here. Could he love Cu? He’d gotten to know him so well… but Cu wasn’t interested in forcing him. Diarmuid was at the start of his own legend, and Cu seemingly at the end of his—destined to die young, instead cursed into a monster. Hesitantly, Diarmuid leaned up to kiss his lips, and surprisingly, Cu let him.

Cu, Cu _, Cu_ , man of myth and legend, one of the early stories that inspired him to become a knight himself, the mad hound of Ulster, beneath his grasp and letting him kiss him. This was supposed to be a quick job, slay the beast and return a hero, but Diarmuid couldn’t bring himself to do it as Cu’s kiss turned rough in return, biting his lower lip with those sharp teeth and invading his mouth with his tongue. Maybe he just wanted to be free and was using Diarmuid to that end, but truly, did he really care? Perhaps Diarmuid’s opinion of himself wasn’t high enough to care about being used for love, or maybe he liked the monster too much to think of it that way.

Each moment that passed, Diarmuid ceded more control to Cu, who dragged his claws up and down his body, leaving raking lines against his back. Diarmuid had to be mindful of thorns digging into his leg as he pressed against him, unafraid and welcoming of every touch Cu had to offer. If a kiss would break the curse, surely he would have been back to normal by now. Diarmuid could tell it needed an extra touch.

His hand warned Cu of its destination by trailing down his waist and was met with no resistance. Cu’s cock was heavy in his hand, heavier than he’d anticipated. If they were going to have sex, then—no, that’s best saved for when it actually happened. He stroked along it, urging it into an erection as Cu absolutely reciprocated that intensity in a kiss. It seemed the further he went along, the more fierce Cu got.

Casually disregarding anything that might interrupt them, Diarmuid pressed his own erection against his, frotting against it as his hands, slick with water, pushed them close together as he was unable to resist moving his hips. This may only be momentary, a whim he followed in the hopes it changed something between them, but Cu was reciprocating, so what else could he do?

“Cu…”

Cu said nothing, he merely let him continue as he worked bites into Diarmuid’s shoulder, rougher and harsher than the rest of his touches, mainly because his teeth were so sharp and ghastly. It reminded Diarmuid that the man he was sitting on was nothing short of a monster, but in Diarmuid’s own delicate touch, he realized that he didn’t mind. Man or beast, Cu was…

He was…

What was he doing?

Diarmuid blinked with sudden clarity. Then, he found himself asking something.

“Cu, do you really want this?”

“…?”

“Do you want me to be the one to break your curse?” Diarmuid could feel his throat going dry, even as his hands kept moving, but he couldn’t stop himself from speaking.

“What does it matter, who it is?” Diarmuid could have predicted Cu’s answer easily, but it hurt so unexpectedly badly to get it. Cu didn’t care about him, he cared about finally getting back to his old self. Somewhere in that apathetic mind of his was a desire to be free of it, like a depression that had fogged his thoughts yet concealed a distinct _want_. This was all Cu had to have ever wanted, to use someone to that end.

Diarmuid could not continue. He got up off of Cu’s lap and waded to the other side of the shore, wringing his clothes dry and putting them back on.

“I should go. There’s no way I could break your curse in this state,” Of course, instead of meaning ‘go back to the cave’, he meant ‘leave forever and wash his hands of this’. Cu could only stare as he did, feeling a touch of anger rise and fall as soon as it hit him.

He very nearly had it. Someone loved him, and as soon as he admitted he didn’t back, he might as well have sealed his own fate.

\--

Diarmuid gathered his things.

There was precious little he wanted to take with him besides his weapons. Physical memories of this venture would be poor reminders of how badly it went. Diarmuid felt used, even though he could have predicted this outcome a mile away, even thought about it as he got on his lap. He didn’t know what he’d tell the village. Only that it would be better to send another knight to…

… well, this was unfortunate. He couldn’t send another knight to have Cu killed, either. As much as he loathed to admit he held feelings, they were still there. Cu was still too important to him to let him get hurt—or let him hurt one of his fellow knights.

Just as he was thinking about it, Cu had entered the mouth of the cave. He was still dripping wet, his clothes hastily thrown on. At that moment, Diarmuid didn’t know if he’d let him pass or he’d have to fight his way out.

Cu surprised him.

He took him by the shoulders and kissed him, though Diarmuid was quick to back up. “I thought anyone could do this for you?”

“Anyone could,” Cu agreed quietly. “I don’t want ‘anyone’, though.”

“Didn’t you just say it didn’t matter who it was?”

Cu looked at him as if trying to figure out how to word how he felt and coming up empty entirely. “I … misspoke.”

That was quite the admission for someone who claimed to care about nothing. Suddenly he cared about something, and it got Diarmuid to at least slow down and listen to him.

“Everyone ran away without even looking at me. I stopped caring. I stopped… thinking this would change. It gets comfortable to not be able to care about anything, but you remind me of who I once was, and how much better that life was. Even if it was meant to be short.”

“…”

“I’d trade a thousand years where all I am is this for one day where you got to know who I used to be.”

“…” Diarmuid’s expression softened. Cu wasn’t the type to lie about something like this, and so he stepped forward.

“I like you either way. Curse or no curse. Vibrant man of legend, or who you are now.”

“Why?” Cu asked. “What have I given you that you couldn’t get from anyone else?”

That question was a challenge Diarmuid couldn’t ignore. “Because you’re Cu Chulainn. That much will always be true. You think you’ve changed by force, and some parts of you might have, but I can still see the real you, fighting yourself for the right to exist.”

“…” Cu cupped his cheek, and Diarmuid looked away from those intense red eyes for but a moment. There was an understanding between them—that Cu might be stuck like this forever, and yet, Diarmuid didn’t mind. Diarmuid was only acquainted with the legend and who he’d gotten to know over the past few months. The latter left a stronger impression on who Cu really was, versus the idealization that Diarmuid had come to develop. This time, the kiss was not hesitant or curious, instead breaking through the barrier between them.

Cu pushed him back onto the pile of furs that had been used as their bed and loomed over him. He couldn’t explain it, but vigor was starting to pump through his body that he only felt on hunts these days—he felt young, tearing off the hood of his outfit and letting his blue hair frame Diarmuid’s face as he leaned over to kiss him. Diarmuid could only keep up as Cu stripped his skin-tight outfit from him. His hands reached up to frame his face, wrapping his legs around his torso and dragging him in close. Sex might be difficult, but there was a bit of olive oil he brought from the village for cooking that could be used. He pushed Cu’s chest gently to go retrieve it.

Cu’s claws had no place stretching anyone out, so Diarmuid had to do it himself and do it _well,_ despite only knowing of this technique from overhearing it between the hushed voices of women before. As Cu was feverishly nipping at his neck, running his tongue over the raw impressions that his teeth left, Diarmuid tried to focus on getting himself comfortable. Just relaxed his body as much as possible before whispering that he was ready to Cu.

… if only he’d known how ready he wasn’t.

Cu was not deliberately painful, but the difference in sizes between their bodies could not be ignored. Cu was not patient, either. He took every inch he was able to, thrusting in roughly until Diarmuid could take him in wholly. It was not the worst pain he’d ever experienced, but it was far from pleasant. He could endure for Cu’s sake, to give him something he’d sorely needed, but he still placed his hand on Cu’s lower abdomen in order to coax him into stopping for a moment.

He could see how tense Cu was, how holding back was taking a toll on him—eventually, Diarmuid moved his hand to grip the furs around him and give his silent permission for Cu to continue. He drove into Diarmuid with a frenzy only a beast could possess, taking all that Diarmuid had given him and more—to the knight beneath him, it was a test of his endurance, feeling himself grow weak as Cu roughly used his body and dragged his tongue up his chest before resting his teeth on his collar bone. He could teach him to be gentler in time, Diarmuid thought, but for now, he’d let Cu feast on his body.

It wasn’t all pain, though. Cu kept driving at a sweet spot deep within him that couldn’t be ignored. Furthermore, when Diarmuid sighed every time he did, Cu took the cue to angle his thrusts just a bit for his sake. He was, by no means, slowing down but he was invested in seeing his pleased face, taking pride that through the harshness of it all, Diarmuid could still find pleasure with this monstrous body.

Everything in motion, everything aching in just the right ways. Diarmuid gave up all of it—everything to this beast, no, this _man_. Raising his hips so that he could meet his thrusts, desperately kissing him and drawing his face back to his own. Even as he tasted of copper, Diarmuid never wanted his lips to leave his.

Throughout this, Cu did not change at all, at least on the outside. His markings didn’t fade, the tail emerging from his body didn’t recede. There was a spark, though, familiar and grand on the inside. The fog was clearing in his mind for the first time in years. As he rode out his orgasm, he finally took notice of the stressed look on Diarmuid’s face, the barely-tolerating-it expression. Immediately, his hand went to his cheek as a whispered curse escaped his mouth.

“Are you all right?”

Diarmuid’s breathing slowed, his body relaxing. This was the first time Cu had ever shown concern in this way. A faint smile crossed his lips as his trembling stopped.

“I’m fine, I probably just—didn’t prepare right—”

Cu looked down at him, before looking at his claws and sighing. “I’m still the same.”

“The same ‘Cu’? You were always Cu. Do you feel any different?”

…

“A bit. Hard to tell.”

“It’s a start. Did you think you’d shed all of your thorns and become human-looking again?”

“I sort of expected…”

“It’s fine. You are who you are. I’ll accept that no matter what.”

Cu went silent for a moment, figuring this was the best he was going to get—perhaps, in a bid to get him to sway to her whims, that woman had disfigured and corrupted him in hopes that he would do anything to change back, only to stay the way suited to her tastes. Whatever the case, he couldn’t deny he felt a bit better. It was going to take work, effort, and time to truly become himself again if he even could; the years had changed him and some things would always be different.

He cleaned Diarmuid up and laid gently next to him, arm wrapped around his waist as he found himself talking; about the past, about his story, about how things really went instead of word of mouth. Diarmuid would fall asleep in his arms, and only then would he fall silent, letting him rest after all of the hard work he put in.

\--

Cu no longer wished for the life of a hermit.

The next day, he gathered what he valued from his cavern and left it.

He told Diarmuid that he had many things to settle, a life he’d put on hold to live a solitary life, people to see, a teacher who probably thought he had died. When he was done, he’d come back for him. Diarmuid gave him one last kiss goodbye and they went their separate ways for the time being.

It was hard, waiting for him. Diarmuid worried that people would see his thorns and tail and quickly target him as a monster instead of as a once-hero, and he’d be forced to fight for his life. Weeks went by with no word, and no word meant no bad news, but he couldn’t help but keep listening for any updates.

Cu wouldn’t keep him waiting very long, though.

He returned with the usual look—markings trailing down his eyes, the swish of his thorny tail. It seemed nothing was going to break that part of the curse. People couldn’t understand that the handsome Diarmuid Ua Duibhne had taken this man as his partner, but Diarmuid himself had noticed the changes.

He came with something else, too; a very thin, blink-and-you’ll-miss-it smile.


End file.
